1. Field of the Invention
This invention relates to a method and a system for the biological treatment of wastewater.
More specifically, the method and system of the present invention are designed for the biological removal of contamination in the form of organic material, and optionally the biological removal of the nutrients, nitrogen and phosphorus from wastewater.
2. Discussion of the Prior Art
The biological treatment of sewage and other wastewaters is by no means new. Examples of methods and apparatuses for effecting such treatment are described in Canadian Patents Nos. 997,488, issued to B. K. Tholander et al on Sep. 21, 1976 and 1,117,042, issued to M. L. Spector on Jan. 26, 1982, and U.S. Pat. Nos. 2,907,463, issued to D. J. N. Light et al on Oct. 6, 1959; 4,279,753, issued to N. E. Nielson et al on Jul. 21, 1981; 4,430,224, issued to U. Fuchs on Feb. 7, 1984; 4,431,543, issued to Y. Matsuo et al on Feb. 14, 1984; 4,522,722, issued to E. M. Nicholas on Jun. 11, 1985; 4,663,044, issued to M. C. Goronszy on May 5, 1987; 4,798,673, issued to C. Huntington on Jan. 17, 1989 and 4,948,510, issued to M. D. Todd et al on Aug. 14, 1990.
In general, conventional activated sludge processes and modifications thereto have recently been shown to be able to accomplish the objectives set out above, namely the biological removal of contamination from wastewaters, but the cost of specialized clarifiers required to settle and pumps to recycle biological solids is quite high. Moreover, the size, complexity and operating problems encountered with such systems make them unattractive to potential users. In addition, the basic activated sludge process often suffers from poor treatment efficiency and unstable performance because of the continuous flow through characteristics in the final clarifier and the resulting constant sludge management requirements.
As a consequence of the foregoing, during the past ten years improved final clarification and solids removal has been sought by using larger and improved designs for final clarifiers. Moreover, final effluent filtration is often necessary to reliably meet permitted environmental standards.
There has been a recent revival of interest in the sequencing batch reactor (SBR) activated sludge process because of the inherently more efficient batch settling and higher treatment efficiency possible for batch organic contaminants removal with the SBR process compared to conventional continuous flow activated sludge process. The SBR process uses the same vessel for batch mixing, aerobic treatment and quiescent batch settling. Thus, the SBR process eliminates the major cost of dedicated final clarifiers and sludge return pumps necessary for the conventional activated sludge process as well as improving upon solids removal performance.
However, the SBR process has some disadvantages, the principle one being that the process does not operate with continuous flow, but requires intermittent operation for cycles of fill, react, settle, decant and idle. The result can be a much lower volume to use ratio than that achieved using the conventional flow through activated sludge process.
Accordingly, the SBR process is generally not cost effective for flows greater than five to ten MGD. Other disadvantages of the SBR process are that significant head loss occurs from the influent to the final effluent, requiring additional energy and pumping costs, and effluent flow is intermittent which can result in the necessity of multiple units or flow equalization systems to prevent peak loadings and adverse impacts on receiving waters.
Finally, the basic process and design limitations of the SBR process make it difficult to achieve the same high efficiency biological nutrient removal possible using the continuous flow activated sludge process.